Self-lubricating bearing material and plain bearing of such a bearing material

ABSTRACT

A self-lubricating bearing material is described, as well as a plain bearing of such a bearing material, the performance of which, under lubricant-free conditions, is so markedly improved that pv values of up to 6 MPa/m/s are achieved in an average load and speed range. The self-lubricating bearing material comprises a PTFE-containing polymer matrix with fillers comprising PbO and at least one metal fluoride. The PbO content is from 15-55 vol. % and the metal fluoride content is from 0.1-14 vol. %. Preferred metal fluorides are CaF 2 , PbF 2  and MgF 2 . The addition of further fillers such as hard materials, pigments or fibrous material is possible. The proportion of further additives may amount to up to 40 vol. % of the PbO/metal fluoride fillers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage of PCT/DE96/00244 filed Feb. 14,1996 and based, in turn, on German national application 195 06 684.7 ofFeb. 25, 1995 under the International Convention.

FIELD OF THE INVENTION

The invention relates to a self-lubricating bearing material of aPTFE-containing polymer matrix with fillers comprising PbO and metalfluorides, as well as to a plain bearing of such a bearing material.

BACKGROUND OF THE INVENTION

Bearing materials with plastic-based overlays are known as single-layer,two-layer or three-layer composite materials: solid plastic bearings,bearings with an outer metallic backing and directly applied or adheredplastics, bearings with inner wire meshes, as well as three-layerbearings of backing metal, a sintered porous metal layer and a coveringlayer formed in the pores. All these bearings are generally used infields in which the use of externally supplied lubricants is impossibleor undesirable. When in operation, therefore, they must intrinsicallyprovide the lubricants.

Multilayer materials differ from solid plastic materials, for example bya negligible tendency towards cold flow under load, by substantiallybetter heat conductivity and, in connection therewith, by notably higherpossible pv values. The solid plastic materials may, however, beadvantageous in certain cases, e.e. for reasons of cost. Amongthree-layer materials, it is possible to distinguish further betweenthose with overlays based on fluorothermoplastics, such as PTFE, PFA,FEP etc., and other plastics such as PEEK. The latter two groups differin their manner of operation: while, in the case of PTFE-basedmaterials, the bronze intermediate layer is the "active" component ofthe overlay and acts like a filler, the other plastic materials use itonly as an anchoring means. If there is sufficient affinity to the metalbacking, they permit the production of true two-layer materials, butthey may also be applied with the aid of an adhesive layer. On theactive sliding surface the thermoset or thermoplastic material assumesthe supporting role of the bronze. Moreover, bearing materials of filledfluorothermoplastic films adhered to metal or other supports with wiremeshes incorporated in the plastic are known and may likewise be adheredto a metal backing.

Each variant has inherent advantages and disadvantages. However, thethree-layer materials based on fluorothermoplastics, such as PTFE, areuniversally applicable and easy to produce and also exhibit the highestperformance and temperature resistance. In the case of the latter,homogeneous mixtures are produced by means of a plastic dispersion andthe final composite material is produced, after the production of aPTFE/filler paste, by a rolling process and subsequent sintering of thePTFE.

The most commonly used fillers for such materials are lead andmolybdenum sulphide, which produce almost identical performance andwhich may be also used with lubrication. In many cases it would bedesirable to solve structural problems by using maintenance-free,space-saving plain bearings with a PTFE overlay, but this must berejected owing to the upper load limit thereof, which, within an averageload and speed range (0.5-100 MPa and 0.02-2 m/s), lies at a pv value ofapproximately 2 MPa m/s.

A sliding material is known from DE 41 05 657 A1 which comprises abacking metal with a porous metal layer, in which definite pores arepresent, the pores and their surfaces being impregnated and coated withan impregnating coating composition. The impregnating coatingcomposition consists of 0.5-30 vol.-% PFA, EPE or FEP, 5-30 vol.-%metallic lead with a specific surface of from 1000-8500 cm² /g in anaverage particle size, 0.5-30 vol.-% metal oxide, metal fluoride,graphite or the like, ceramics, such as SiC, and fibrous material, suchas carbon fibers or glass fibers, the rest consisting of PTFE. The totalof all components other than PTFE amounts to from 6-50 vol.-%.

DE 41 05 657 A1 mentions only a metal fluoride-containing impregnatingmaterial comprising 2% PbF₂ but no metal oxide, wherein said material isnot distinguished from other exemplary embodiments comprising no metalfluoride by improved cavitation resistance or wear resistance.

EP 0 183 375 A2 describes a bearing especially for shock absorbers,whose overlay consists, for the purpose of improving the cavitationresistance, of PTFE with a fluoride of low water-solubility. Thefluoride content, which may, inter alia, be CaF₂ or MgF₂, is stated asbeing from 1-50 vol.-%, preferably from 10-30 vol. %. 20 vol. % is citedas a preferred example. GB 912,793 describes standard bearing materialsof PTFE with lead and/or lead oxide, wherein the filler content is from16-44%, preferably from 16-30%.

The self-lubricating bearing known from DE 35 16 649 A1 comprises thinlyrolled-out sheet- or scale-like particles in the PTFE matrix, which formsmears which consist of a plurality of layers arranged spacedly oneabove the other and extending substantially parallel to the surface ofthe sintered metal layer. Metal oxides and metal fluorides, inter alia,are cited as solid lubricants, only lead and lead oxide being discussedas Examples.

OBJECT OF THE INVENTION

The object of the invention is to provide a self-lubricating bearingmaterial as well as a plain bearing comprising such a self-lubricatingbearing material, the performance of which is so markedly improved underlubricant-free conditions that pv values of up to 6 MPa m/s are reachedin the average load and speed range.

SUMMARY OF THE INVENTION

This object is attained, in accordance with the invention by providing aself-lubricating bearing material of a PTFE-containing polymer matrixwith fillers comprised of 15 to 55 volume percent PbO and at least onemetal fluoride in an amount of 0.1 to 14 volume percent. The inventionalso relates to a plane bearing with a self-lubricating plastic overlaywhich contains this filler.

The invention is based on the recognition that performance underlubricant-free conditions may be markedly increased when a certainfiller combination comprising PbO and metal fluoride is used inconjunction with PTFE. It is important to keep this filler combinationof PbO and metal fluoride within a narrow range.

It has surprisingly emerged that, for PTFE-containing materials, a rangeof from 0.1-14 vol. % is particularly excellent in the case of metalfluorides, which was not previously known from the prior art. AlthoughEP 0 18 33 75 mentions a metal fluoride content of from 1-50% and arelatively high content of 20% is described therein as particularlyadvantageous, this is obviously because in the known sliding materialabsolutely no PbO is present, such that the lack of this filler has tobe compensated by a high metal fluoride content. The fact that a metalfluoride content of from 0.1-14 vol. % results in particularlyadvantageous properties is presumably the consequence of the interactionwith lead oxide, which must at the same time be included in the slidingmaterial in an amount ranging from 15-55 vol. %. Above 14 vol. % nofurther improvement can be achieved by increasing the metal fluoridecontent with respect to PbO. Although according to DE-OS 41 05 657 PbOand metal fluoride may be jointly present, the PbO and metal fluoridecontents according to the invention have not always been recognized asparticularly advantageous, because the improved properties of thesliding material described therein results substantially from themetallic lead additionally present and its particularly large specificsurface of up to 8500 cm² /g. It has been shown that, when the PbO andmetal fluoride contents according to the invention are maintained, anadditional lead content may be completely dispensed with.

In PTFE matrix materials a metal fluoride, especially calcium fluoride,content of 0.1-14 vol. % has proven especially advantageous. However, itwas also possible to achieve marked improvements in the slidingmaterials within this range with PbF₂, MgF₂ or another fluorocompound ofan element from the second main Group of the Periodic Table.

Even if the above-described advantageous mixtures of the fillercombinations according to the invention and PTFE are integrated intoother polymer matrices such as those, for example, of PEEK(polyetherether ketone), PPS (polyphenylene sulphone), PA (polyamide),PVDF (polyvinylidene fluoride), PSU (polysulphone), PES (polyethersulphone) or PEI (polyether imide), improved results are achieved incomparison to variants filled only with PTFE or with PTFE and PbO.However, the best results were achieved with PTFE in conjunction withthe filler combinations according to the invention. If the polymermatrix consists of PTFE, within the range according to the invention offrom 0.1-14 vol. % CaF₂ and 15-55 vol. % PbO the range of from 24-40vol. % PbO and 0.3-8 vol. % CaF₂ is particularly advantageous.

The improved properties of the sliding material achieved with the fillercombination according to the invention are retained or could be improvedfor specific applications by further additives, if the proportion ofthese further additives amounts at most to 40 vol. %, preferably to 20vol. %, of the PbO/metal fluoride filler combination. Further additivesmay be those from one or more of the group comprising hard materials,pigments, fibrous materials, solid lubricants or thermoset or hightemperature thermoplastics materials. Particularly suitable hardmaterials are Si₃ N₄ and particularly suitable pigments are Fe₂ O₃ orcarbon black. Particularly suitable fibrous materials are short graphitefibers or aramid fibers. Preferred solid lubricants are MoS₂ and boronnitride. Polyamide imides or polyimides may be used as thermoset or hightemperature-resistant thermoplastics materials.

A plain bearing with such an overlay may be constructed as a solidplastics bearing or with a metal backing. The overlay may be applieddirectly to a metal backing or a layer of porous sintered bronze may beprovided on the metal backing, on which the overlay is then applied.

The three-layer materials may, for example, be constructed in such a waythat a 0.15-0.5 mm thick layer of bronze is sintered onto the backingmetal, such as steel or a copper or aluminum alloy for example, in sucha way that it has a pore volume of from 20-45% and the bronze iscomposed of from 5-15% tin and optionally additionally 5-15% lead. Theplastics mixture is then applied by rolling onto the porous backing insuch a way that the pores are completely filled in and, depending on theapplication, a 0-50 μm covering layer arises. Thereafter, the materialis subjected to heat treatment in a furnace, wherein the PTFE obtainedis sintered for 3 mins at 380° C. to produce the final compositematerial and the necessary final dimensions in a final rolling stage.

Production of the plastics mixture may be effected by means of a PTFEdispersion, which is added to the fillers in such a way that they areentrained in homogeneous dispersion when subsequent coagulation iscarried out. A pasty composition

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become morereadily apparent from the following description, reference being made tothe accompanying drawing in which:

In the drawings:

FIG. 1 is a diagram in which service life in dependence on the CaF₂content is represented graphically; and

FIGS. 2 through 5 are diagrams in which the rate of wear or local limitfor various compositions is represented graphically.

SPECIFIC DESCRIPTION AND EXAMPLES Example 1

9 l water, 20 g sodium lauryl sulphate, 13.4 kg PbO, 0.85 kg CaF₂ and21.6 kg of a 40% PTFE dispersion are stirred energetically for 20 mins.500 g of a 20% aluminum nitrate solution are then added. After thecompletion of coagulation, the consistency necessary for processing isproduced by stirring 1 l of toluene into the mixture for 10 mins.

                  TABLE 1                                                         ______________________________________                                                                   Wear    Friction                                   Ex. No.  Composition, vol. %                                                                             [μm/h]                                                                             coefficient                                ______________________________________                                        0        80 PTFE, 20 Pb (comparison                                                                      120     0.20                                                material, prior art)                                                 1        70 PTFE, 25.5 PbO, 4.5 CaF.sub.2                                                                29      0.17                                       2        70 PTFE, 30 PbO   40      0.17                                       3        65 PTFE, 26.2 PbO, 8.8 CaF.sub.2                                                                26      0.16                                       4        65 PTFE, 35 PbO   38      0.15                                       5        60 PTFE, 35 PbO, 5 CaF.sub.2                                                                    20      0.15                                       6        60 PTFE, 40 PbO   35      0.17                                       ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                                   Wear    Friction                                   Ex. No.  Composition, vol. %                                                                             [μm/h]                                                                             coefficient                                ______________________________________                                        5 (for com-                                                                            60 PTFE, 35 PbO, 5 CaF.sub.2                                                                    20      0.15                                       parison)                                                                      7        60 PTFE, 31.5 PbO, 4.5 CaF.sub.2,                                                               15      0.17                                                4 Si.sub.2 N.sub.4                                                   8        60 PTFE, 29.8 PbO, 4.2 CaF.sub.2,                                                               13      0.18                                                6 C fibers                                                           9        60 PTFE, 33.3 PbO, 4.7 CaF.sub.2,                                                               18      0.15                                                2 Fe.sub.2 O.sub.3                                                   10       60 PTFE, 30.8 PbO, 4.4 CaF.sub.2,                                                               8       0.16                                                4.8 MoS.sub.2                                                        11       60 PTFE, 32.2 PbO, 4.6 CaF.sub.2,                                                               26      0.20                                                3.2 PAl                                                              ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                                   Wear    Friction                                   Ex. No.  Composition, vol. %                                                                             [μm/h]                                                                             coefficient                                ______________________________________                                        3 (for com-                                                                            65 PTFE, 26.2 PbO, 8.8 CaF.sub.2                                                                26      0.16                                       parison)                                                                      12       65 PTFE, 26.2 PbO, 8.8 MgF.sub.2,                                                               30      0.17                                       13       65 PTFE, 26.2 PbO, 8.8 PbF.sub.2,                                                               28      0.15                                       14       80 PEEK, 20 PTFE  36      0.20                                       15       80 PEEK, 12.5 PTFE, 7.5 PbO                                                                     16      0.15                                       16       80 PEEK, 12 PTFE, 7.2 PbO,                                                                      10      0.17                                                0.8 CaF.sub.2                                                        ______________________________________                                    

It has been shown that the compositions according to the invention aremarkedly superior to the standard materials based on PTFE/MoS₂ orPTFE/Pb as far as both the friction coefficient and wear resistance areconcerned and even surpass the compositions of PTFE and PbO which arebetter than these sliding materials.

The PTFE, PbO and CaF₂ composition was varied over a large range andsamples of the above-described three-layer materials were producedcomprising 1.25 mm steel, 0.23 mm bronze and a 0.02 mm plastic coveringlayer. These samples were measured by means of a pin/roller tribometer,using test pieces of 0.78 cm², with respect to rates of wear at acircumferential speed of 0.52 m/sec and a load of 17.5 MPa and comparedwith a standard material. The standard material used was a compositematerial with a plastics layer of 80 vol. % PTFE and 20 vol. % lead,which is cited as Example No. 0 in Table 1 and designated in FIGS. 1, 2and 4 as a standard or material according to the prior art.

FIG. 1 shows the service life of a plain bearing according to theinvention in dependence on the CaF₂ filler content, the total fillercontent being kept constant at 40 vol. %. It may be clearly seen thatthe hatched area represents a clearly pronounced maximum with respect toservice life, corresponding to minimum wear. In the maximum area theservice life values reached are approximately three times those whichare achieved with a bearing material comprising only lead oxide in thepolymer matrix. To clarify the improvements which may be obtainedaccording to the invention, the material compositions given in Table 1together with the friction coefficients and rates of wear from thepin/roller test were tested. In FIG. 2 the results are graphicallycompared. It may be clearly seen that the sliding materials with thefiller combination PbO/CaF₂ does better in all cases than a slidingmaterial comprising only PbO.

The results of Examples 7 to 11 compared in Table 2 and FIG. 3 show thatthe sliding materials according to the invention may also be combinedwith further additives without the positive properties being lost. Theseadditives sometimes even result in marked improvements.

In Examples 12 and 13 magnesium fluoride and lead fluoride were usedinstead of calcium fluoride. The corresponding results, compared with acomposition comprising calcium fluoride, are summarized in Table 3. Itis clear that other fluorocompounds including metals from Group II ofthe Periodic Table or lead fluoride are also capable of achieving theeffect according to the invention.

Bushings with a diameter of 22 mm were produced from the compositionsaccording to Examples 5 and 6 and their load limits were tested in arotating test run. The load limit was defined as the highest possibleload under which a running distance of 22.5 km was achieved at a speedof 0.125 m/sec. The failure criterion was a considerable increase infriction coefficient and temperature, which proved in a subsequentinvestigation to be synonymous with an average wear depth of 90 μm. Theresult of Example 5 corresponds, when evaluated in a different way, to apv value of 6 MPa.m/sec. The corresponding results are representedgraphically in FIG. 4.

Another possible way of advantageously carrying out the inventionconsists in working the mixtures according to the invention, includingPTFE, into a thermoplastics matrix and then processing the latter in anydesired manner to form a sliding element, e.g. applying it to a metalbacking with or without a bronze intermediate layer or producing solidplastics parts. The thermoplastics content may vary between 60 and 95vol. %. Examples 2 to 13 were produced in the same way. In Examples 14to 16 PEEK and PTFE were used instead of the matrix material PTFE andthe samples were produced by powder mixing. The mixtures may also beproduced by extrusion compounding. To clarify the effect according tothe invention, Table 4 and FIG. 5 show the friction and wearcoefficients of PEEK compounds with PTFE, PTFE/PbO and PTFE/PbO/CaF₂. Itmay be seen that the CaF₂ here also supports the wear-reducing effect ofthe lead oxide.

We claim:
 1. A self-lubricating bearing material of a PTFE-containingpolymer matrix with fillers comprising PbO and calcium fluoride,whereinthe PbO content amounts to from 15 to 55 vol. % and the calciumfluoride content amounts to from 0.1 to 14 vol. %.
 2. A bearing materialas claimed in claim 1, wherein, in addition to PTFE, the polymer matrixalso comprises PEEK, PPS, PA, PVDF, PSU, PES or PEI individually or incombination.
 3. A bearing material as claimed in claim 1, wherein thepolymer matrix consists of PTFE and the fillers consist of from 15 to 55vol. % PbO and 0.1 to 14 vol. % CaF₂.
 4. A bearing material as claimedin claim 3, wherein the polymer matrix consists of PTFE and the fillersconsist of from 20 to 40 vol. % PbO and 0.3 to 8 vol. % CaF₂.
 5. Abearing material as claimed in claim 1, wherein the fillers include hardmaterials, pigments, fibrous materials, solid lubricants or thermoset orhigh temperature thermoplastic materials.
 6. A bearing material asclaimed in claim 5, wherein the hard materials are Si₃ N₄.
 7. A bearingmaterial as claimed in claim 5, wherein the pigments are Fe₂ O₃ orcarbon black.
 8. A bearing material as claimed in claim 5, wherein thefibrous material is short graphite fibers or aramid fibers.
 9. A bearingmaterial as claimed in claim 5, wherein the solid lubricants are MoS₂ orboron nitride.
 10. A bearing material as claimed in claim 5, wherein thethermoset or high temperature thermoplastic materials are polyamideimides or polyimides.
 11. A plain bearing with a self-lubricatingplastic overlay, whereinthe plastic overlay is metallic-lead free andcomprises PTFE-containing polymer matrix and from 15 to 55 vol. % PbOand 0.1 to 14 vol. % calcium fluoride as fillers.
 12. A plain bearing asclaimed in claim 11, wherein the overlay is applied directly to a metalbacking.
 13. A plain bearing as claimed in claim 11, wherein the overlayis applied to a metal backing provided with a layer of porous sinteredbronze.
 14. A plain bearing as claimed in claim 13, wherein the sinteredbronze has a pore volume of from 20 to 45%.
 15. A plain bearing asclaimed in claim 13, wherein the sintered bronze comprises from 5 to 15%tin.
 16. A plain bearing as claimed in claim 13, wherein the sinteredbronze comprises from 5 to 15% tin and from 5 to 15% lead.